In modern processors and other semiconductor devices, it is known that as the product ages, certain degradations become manifest. Several different phenomena can cause degradation to a semiconductor device, for example, hot-carrier injection, bias temperature instability, oxide breakdown (also known as time dependent dielectric breakdown (TDDB)), electro-migration and more. Each of these degradation mechanisms occurs due to various factors like temperature, voltage, current and others. For example, a frequency degradation occurs over a product's lifetime due to negative bias temperature instability (NBTI) degradation. This degradation becomes a reliability issue for p-channel metal oxide semiconductor (PMOS) and n-channel metal oxide semiconductor (NMOS) transistors. NBTI manifests itself as an increase in the threshold voltage and consequent decrease in drain current and transconductance. The degradation is caused by temperature and voltage applied to the product over time, where the temperature and voltage impact the degradation exponentially.
One manner of handling this issue is by applying a voltage/frequency guard band at a product's beginning of life (when the product is non-aged). However, this guard band limits performance for much of the useful life of the device. For example, maximum frequency (Fmax) and minimum voltage (Vmin) settings at a beginning of life (when the product is fresh) are set assuming end of life degradation (when the product is aged). As a result, there is a speed guard band as the frequency is set lower than a maximum rated frequency of the unit at a fixed voltage and/or the voltage is set higher than a minimum rated voltage of the unit at a fixed frequency.
No dynamic mechanism exists today to control the rate of aging in a semiconductor device. Consequently, static assumptions are made about the expected rate of aging, thereby causing an increase in operating voltage (as discussed above). It would therefore be beneficial to control the rate of aging (e.g., by controlling the operating conditions of the processor), to reduce operating guard bands without the need to set static limits.